Ammonia Electro-oxidation Research Articles

Progress of ammonia electro-oxidation catalysts for direct ammonia alkaline fuel cells

At present, hydrogen fuel cells are one of the effective means to solve energy and environmental problems, but the problem of hydrogen storage and transportation is difficult to break through. Ammonia is a carbon-free hydrogen-rich compound, which also has the advantages of high bulk density and easy storage and transportation, and is an ideal fuel to replace hydrogen as a battery. However, at present, the preferred ammonia electro-oxidation catalyst for direct ammonia alkaline fuel cells is still platinum, and its expensive price is the main problem that hinders the wide application of direct ammonia alkaline fuel cells. Therefore, the design of low-cost and high-efficiency catalysts is a necessary task for commercializing direct ammonia fuel cells. In this paper, we firstly review the progress of researchers in ammonia electrooxidation mechanism in recent years, focus on reviewing the current research status of Pt catalysts and non-Pt catalysts for direct ammonia alkaline fuel cells, and finally suggest future research approaches to address the existing problems, aiming to provide ideas for designing more efficient, stable and cheap ammonia electrooxidation catalysts in the future.

CuCr Bimetallic Catalyst for Selective Electrooxidation of Ammonia at Room Temperature

AbstractIn an effort to attain a low‐cost, energy‐efficient, and non‐noble electro‐catalyst that selectively oxidises ammonia to nitrogen at room temperatures, bimetallic carbon‐supported CuxCry (x,y=1–9) catalysts were prepared and found to be more active than elemental Cu and Cr towards ammonia oxidation (AOR). Cyclic voltammetry (CV) measurements revealed an increase in activity with Cu content. Peak current density of 90 % Cu catalyst in 0.1 M KOH+0.1 M NH4OH was found to be 22 mA/mg (scan rate=20 mV/s) compared with 14.8 mA/mg for commercial Pt catalyst under the same conditions. Mass spectrometry measurements have identified higher selectivity of this catalyst towards N2 than on Cu alone. Pre‐oxidation of catalyst in 0.1 M KOH (1.1 V vs RHE) was found to improve the activity and stability towards AOR over time. The potential of CuxCry (x, y=1–9) based catalysts for AOR in fuel cells is corroborated.

Autonomous electrochemical system for ammonia oxidation reaction measurements at the International Space Station

An autonomous electrochemical system prototype for ammonia oxidation reaction (AOR) measurements was efficiently done inside a 4'' x 4'' x 8'' 2U Nanoracks module at the International Space Station (ISS). This device, the Ammonia Electrooxidation Lab at the ISS (AELISS), included an autonomous electrochemical system that complied with NASA ISS nondisclosure agreements, power, safety, security, size constrain, and material compatibility established for space missions. The integrated autonomous electrochemical system was tested on-ground and deployed to the International Space Station as a “proof-of-concept” ammonia oxidation reaction testing space device. Here are discussed the results of cyclic voltammetry and chronoamperometry measurements done at the ISS with a commercially available channel flow-cell with eight screen-printed electrodes, including Ag quasi-reference (Ag QRE) and carbon counter electrodes. Pt nanocubes in Carbon Vulcan XC-72R were used as the catalyst for the AOR and 2 μL drop of Pt nanocubes/ Carbon Vulcan XC-72R, 20 wt%, ink was placed on the carbon working electrodes and allowed to dry in air. After the AELISS was prepared for launch to the ISS, a 4 days delayed (2 days in the space vehicle Antares and 2 days space transit to the ISS) cause a slight shift on the Ag QRE potential. Nevertheless, the AOR cyclic voltametric peak was observed in the ISS and showed ca. 70% current density decrease due to the buoyancy effect in agreement with previous microgravity experiments done at the zero-g aircraft.

Efficient electrooxidation of ammonia using platinum supported on titania/sulfur-doped onion like carbon nanoparticles in direct ammonia fuel cells

Efficient electrooxidation of ammonia using platinum supported on titania/sulfur-doped onion like carbon nanoparticles in direct ammonia fuel cells

Green Chemistry: Advanced Electrocatalysts and System Design for Ammonia Oxidation

The hazards associated with handling hydrogen fuels have driven people to consider alternative clean and sustainable fuel types. Ammonia shows significant potential for this task as both a direct fuel and hydrogen carrier due to its unique features of facile transportation and low cost. Regarding this, electrochemical ammonia oxidation reaction (AOR) is the essential process for utilizing ammonia for energy applications, either for hydrogen production via ammonia splitting or energy generation via direct ammonia fuel cells, which is highly commercially promising. On this basis, the development of high‐performance and economic electrocatalysts for AOR is critical. In this review, the kinetics and mechanism of ammonia electrooxidation are first discussed to provide a foundation to understand the current issues associated with this technology, and then a comprehensive presentation on the different types of electrocatalysts for AOR is illustrated. Afterward, an outlook is presented and the possible research directions for AOR electrocatalysis are proposed, which is expected to shed light on the future development of this promising technology.

Regulating Competitive Adsorption on Pt Nanoparticles by Introducing Pb to Expedite Hydrogen Production via Ammonia Oxidation

Ammonia with a high hydrogen content is an ideal hydrogen carrier due to its mature storage and transport system. Ammonia electrooxidation is one of the most promising technologies for green and economic H2 production. However, ammonia oxidation reaction (AOR) is limited to a narrow potential window due to the slow kinetics and the competitive adsorption between the reactant and Had/OHad on active sites. Herein, an ultra-small PtPb alloy nanocatalyst is synthesized for AOR by the facile one-step solvothermal method. Experimental results demonstrate that the electronic structure of Pt is regulated by the introduction of Pb, inhibiting the competitive adsorption on Pt sites. Therefore, it exhibits improved AOR activity with a peak current density of 191.2 mA mg–1Pt at 10 mV s–1, which is 1.97 times that of Pt/C. The good performance is attributed to the high hydrogen evolution overpotential property of Pb and the strong p–d electron interaction of PtPb, which effectively modulates the adsorption/desorption of intermediates at the Pt site. Hence, this work proposes a simple and effective strategy for designing efficient AOR catalysts, promoting the development of H2 production by AOR.

Self-Terminating Surface Reconstruction Induced by High-Index Facets of Delafossite for Accelerating Ammonia Oxidation Reaction Involving Lattice Oxygen.

Ammonia (NH3 ) is a promising hydrogen (H2 ) carrier for future carbon-free energy systems, due to its high hydrogen content and easiness to be liquefied. Inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) are desired in whole ammonia-based energy systems. In this work, ultrasmall delafossite (CuFeO2 ) polyhedrons with exposed high-index facets are prepared by a one-step NH3 -assisted hydrothermal method, serving as AOR pre-catalysts. The high-index CuFeO2 facet is revealed to facilitate surface reconstruction into active Cu-doped FeOOH nanolayers during AOR processes in ammonia alkaline solutions, which is driven by the favorable Cu leaching and terminates as the 2p levels of internal lattice oxygen change. The reconstructed heterostructures of CuFeO2 and Cu-doped FeOOH effectively activate the dehydrogenation steps of NH3 and exhibit a potential improvement of 260mV for electrocatalytic AOR at 10mA cm-2 compared to the pre-restructured phase. Further, density functional theory (DFT) calculations confirm that a lower energy barrier of the rate-determining step (*NH3 to *NH2 ) is presented on high-index CuFeO2 facets covered with Cu-doped FeOOH nanolayers. Innovatively, lattice oxygen atoms in Fe-based oxides and oxyhydroxide are involved in the dehydrogenation steps of AOR as a proton acceptor, broadening the horizons for rational designs of AOR catalysts.

Unveiling the particle size effect and surface reactivity of Pt/C nanoparticles for ammonia electrooxidation using in situ infrared spectroscopy

In situ PM-IRRAS provides information about the reaction products of ammonia electrooxidation on the surface of Pt/C nanoparticles and in the bulk of electrolyte.

A CoOxHy/β-NiOOH electrocatalyst for robust ammonia oxidation to nitrite and nitrate

In this work, we describe a composite electrocatalyst for the electrooxidation of ammonia to nitrite and nitrate under ambient conditions as a step towards stable, practical materials for this process.

Modeling and Optimizing Anode Catalyst Layer for Direct Ammonia Fuel Cell

Great progress has been made in recent years in the development of low-temperature direct ammonia fuel cells (DAFCs), motivated by the recognition that ammonia is a carbon-free hydrogen carrier with high energy density, low production cost, and ease in liquefaction at ambient temperature. However, the sluggish kinetics of ammonia electrooxidation and especially complicated mass transport in the anode catalyst layer hinder the further development of DAFCs. In this work, a three-dimensional two-phase multicomponent DAFC model considering the effect of ammonia crossover has been developed. Maxwell-Stefan model, Darcy's law and Brinkman equation are utilized to simulate the multicomponent fluid motion and transport. The predicted polarization curve simulates all experimental results well. The model shows the rate of ammonia crossover decreases with the increase of current density. Besides, the effects of the physicochemical property of the anode catalyst layer, including porosity, thickness and PtIr loading, on cell performance are investigated. The modeling results indicate that decreasing porosity and increasing thickness can slightly improve the electrochemical performance of DAFCs at high current density. Meanwhile, higher PtIr loading can effectively reduce voltage loss before approaching the limiting current density.

Preparation of nickel-copper layered double hydroxide on nickel foam as a non-noble catalyst for efficient electrooxidation of ammonia

Electrochemical oxidation of ammonia is an energy-effective and environment-friendly technology for ammonia-containing wastewater treatment. The key challenge is to explore the high activity and low-price catalyst for ammonia electrooxidation. NiCu layered double hydroxide (NiCu-LDH) on nickel foam (NF) was synthesized for ammonia oxidation via a facile hydrothermal reaction. The Ni0.9Cu0.1-LDH/NF achieved a maximum current density of 78 mA·cm−2 at 1.76 V vs. RHE and a lowest Tafel slope of 195.64 MV·dec−1 in comparison with other catalysts, which was prepared by using different molar ratios of Ni2+ and Cu2+. The Ni0.9Cu0.1-LDH/NF showed smaller and denser nanosheets compared with that of Ni(OH)2/NF, indicating it had more electrochemical active areas and active sites. Chronoamperometry tests showed that the Ni0.9Cu0.1-LDH /NF reached a current density of 23 mA·cm−2 after 120 mins, which was four times as high as Ni(OH)2/NF (6 mA·cm−2). The above results demonstrated that incorporating Cu into the Ni-based layered hydroxides could improve the electrochemical activity of the catalyst, showing Ni0.9Cu0.1-LDH /NF to be a prospective inexpensive, and steady catalyst for high-performance of ammonia electrooxidation.

Theoretical study of transition metal atom pairs anchored in g-CN monolayers for ammonia decomposition

One way to efficiently store, transport, and utilize hydrogen is to convert it into liquid ammonia (NH3). Exploring low-cost and high-efficiency electrocatalysts for liquid ammonia oxidation reaction (AOR) is critical in developing hydrogen production fuel cells. Here, we have investigated the catalytic electro-oxidation of liquid ammonia on surfaces of transition metal dimer anchored in g-CN (TM2@g-CN) monolayer to derive insights into the reaction mechanism and evaluate the catalystic activity. Our results show that the mechanism proposed by Gerischer and Mauerer is kinetically preferred. Furthermore, Fe2, Co2, Ru2, Rh2, and Ir2 anchored in g-CN monolayer exhibit high AOR catalytic activity. In particular, Rh and Ir atoms exhibit excellent performance for hydrogen evolution reaction (HER), indicating that they can be used as the efficient bifunctional catalysts towards ammonia splitting for production H2. Remarkably, by regulating TM atoms with different d-electron numbers, the d-band center (εd) of TM atoms on TM2@g-CN can be turned and utilized to predict AOR performance, which provides a theoretical guideline for the design of advanced AOR electrocatalysts.

Metal Oxide Inclusion in Polycrystalline Pt-Based Electrocatalyst for an Ammonia Oxidation Reaction

In April 2021, an average daily amount of 421 ppm of carbon dioxide (CO2) was recorded in the atmosphere, CO2 has increased more than 100 ppm in the last 66 years. Fuel cells are a suitable option for addressing the major issues of meeting current and future energy demand while also reducing CO2 pollution. If the fuel source originates from renewable sources, this makes the overall process CO2-neutral. In the quest for sustainable fuels with high energy density, focus is currently on nitrogen-containing fuels, such as ammonia (NH3) since their oxidation will yield predominantly dinitrogen (N2). Different platinum precursors (K2PtCl6 and H2Pt(OH)6) were used to study the effect on the electrocatalytic activity of Pt for an ammonia oxidation reaction when different metal oxides are added to Pt particles. The metal oxides chosen are CeO2, Fe2O3, and Ag2O. The morphology and element distribution of the Pt catalyst was determined by SEM equipped with an EDS detector. The crystalline phase present in the samples was determined by X-ray diffraction. Cyclic voltammetry was used to characterize the particles and to study the catalytic activity of Pt electrocatalyst for ammonia oxidation. No significant difference was found (by student t-test) between the onset potential for ammonia electrooxidation of Pt particles and Pt precursor. Average onset potential for ammonia electrooxidation was -0.464 V vs Ag|AgCl. The highest average current peak densities were reported by catalyst from non-chlorinated precursor with addition of Fe2O3. Current densities of nanoparticles synthesized from the H2Pt(OH)6 precursor decreased to 24.9 x 10-2 mA/cm-2 with CeO2 addition, but increased to 28.2 x 10-2 mA/cm-2 when Fe2O3 was added. We are currently working on the electrochemical characterization and oxidation of ammonia with catalyst from Ag2O addition

Sulfur induced surface reconfiguration of Ni1Cu3-S-T/CP anode for high-efficiency ammonia electro-oxidation

As an effective method of removing ammonia from wastewater, ammonia electro-oxidation technology has emerged as an attention treatment owing to its simple operation and better tolerance to toxic pollutants. Notably, how to solve the sluggish kinetics of ammonia oxidation reaction (AOR) is the key to the technology. In this work, a high-efficiency catalytic electrode was obtained by deepening the surface electrochemical reconfiguration of NiCu nanoparticles through sulfur-doping. Electrochemical test showed that the Ni1Cu3-S-T/CP electrode had a maximum current density of 150 mA/cm2 at 1.69 V vs RHE. Density functional theory calculations further elucidated that the proper introduction of copper optimized the electron band structure and ameliorated ammonia adsorption and desorption capacity, forcing the early start of AOR at low potential. Consequently, after 5 h electrolysis at 1.50 V, the removal efficiency of NH4+-N was 96.23 %. As well as the electrode maintained good stability in 12 cycles tests.

A Survey of Catalytic Materials for Ammonia Electrooxidation to Nitrite and Nitrate.

Studies of the ammonia oxidation reaction (AOR) for the synthesis of nitrite and nitrate (NO2/3 - ) have been limited to a small number of catalytic materials, majorly Pt based. As the demand for nitrate-based products such as fertilisers continues to grow, exploration of alternative catalysts is needed. Herein, 19 metals immobilised as particles on carbon fibre electrodes were tested for their catalytic activity for the ammonia electrooxidation to NO2/3 - under alkaline conditions (0.1 m KOH). Nickel-based electrodes showed the highest overall NO2/3 - yield with a rate of 5.0±1.0 nmol s-1 cm-2 , to which nitrate contributed 62±8 %. Cu was the only catalyst that enabled formation of nitrate, at a rate of 1.0±0.4 nmol s-1 cm-2 , with undetectable amounts of nitrite produced. Previously unexplored in this context, Fe and Ag also showed promise and provided new insights into the mechanisms of the process. Ag-based electrodes showed strong indications of activity towards NH3 oxidation in electrochemical measurements but produced relatively low NO2/3 - yields, suggesting the formation of alternate oxidation products. NO2/3 - production over Fe-based electrodes required the presence of dissolved O2 and was more efficient than with Ni on longer timescales. These results highlight the complexity of the AOR mechanism and provide a broad set of catalytic activity and nitrate versus nitrite yield data, which might guide future development of a practical process for the distributed sustainable production of nitrates and nitrites at low and medium scales.

Catalysts for electrochemical ammonia oxidation: Trend, challenge, and promise

Developing sustainable and clean energy technology is critial to address the current energy and environmental crisis. As an energy carrier, hydrogen has been getting attention due to its abundance, high energy density, and zero carbon emission. Due to the difficulty of hydrogen storage and transport, the electro-oxidation of ammonia shows great potential for renewable energy since it satisfies both energy supply and environmental protection as a carbon-free fuel with high hydrogen content. However, ammonia electro-oxidation is a sluggish reaction and shows low durability. Understanding the complicated mechanism and intermediates is conducive to developing efficient electrocatalysts with high catalytic activity and durability. This review begins with the ammonia oxidation technologies and their principles. The two typical mechanisms for ammonia electro-oxidation, Oswin-Salomon mechanism and Gerischer-Mauerer mechanism, are discussed, and attention to the poisoning species is included. Then, the electrocatalysts for ammonia reaction are classified into Pt-based catalysts and non-noble metal-based catalysts, and their performances are reviewed with various strategies. Based on the progress on the electrochemical ammonia oxidation catalysts, the challenges and future insight on ammonia oxidation reaction are discussed and proposed.

(Energy Technology Division Graduate Student Award sponsored by BioLogic) Voltage as a Driving Force for Ammonia Activation

In chemical synthesis, the making and breaking of chemical bonds often requires traversing large energy differences. In fact, in 2012 the basic chemical industry accounted for close to 20% of total delivered energy consumption in the industrial sector, which itself used the most delivered energy of any end-use sector globally (54%). Traditionally, industrial chemical synthesis has relied on pressure and temperature to drive chemical reactions, and the energy to elevate the pressure and temperature generally comes from fossil fuel sources. These chemical reactors also often require centralized locations so that economies of scale make them profitable (e.g., ammonia synthesis). However, with the advent of distributed and accessible renewable electricity, it is attractive to consider driving chemical reactions that are conventionally driven with temperature and pressure with renewable electricity instead. Electrochemical reactors can be driven at mild temperatures and pressures and can take advantage of renewable electricity sources directly, enabling decentralized manufacturing. Thus, electrification of the chemical industry offers an opportunity to further integrate the chemical and energy industries, motivating and leveraging decentralized renewable energy sources and storage for chemical manufacturing. In this talk, I will first look at the broad question of how to compare electrochemical routes with traditional thermochemical routes for chemical transformations, aiming to answer the question “If I can apply mechanical energy (pressure), thermal energy (temperature), or electrical energy (voltage) to a chemical reaction, which should I use?” I will present a framework for comparing voltage, temperature, and pressure as thermodynamic driving forces to help quantitatively discriminate between energy sources. Second, I will discuss electrochemical utilization of ammonia. Ammonia has one of the largest global production rates by volume, and it is a nexus synthesis molecule: it either directly or indirectly provides nitrogen for a range of molecules including specialty chemicals, polymers, and pharmaceuticals. Ammonia is also attractive as an energy-dense, carbon-free fuel. Because of this, it represents an important target for electrification. I will discuss the kinetics of ammonia electro-oxidation, i.e., the breaking of the nitrogen-hydrogen bonds, as well as how an applied potential can help form carbon-nitrogen bonds, an electrochemical analogue to traditional reductive amination. Last, I will propose an energy storage paradigm that leverages ammonium formate, a combination of ammonia and formic acid, to store renewable electricity. I will discuss the advantages of this fuel and demonstrate how voltage can aid in the release of energy from this fuel. Overall, I will start with the broad question of why and when to use voltage in the chemical transformations, and then I will focus on how electrochemistry can aid in ammonia utilization for both synthesis reactions and energy storage purposes.

γ-Alumina Supported Copper Oxide Nanostructures Promoted with Ruthenium Oxide (RuO2-CuO/Al2O3) and Palladium Oxide (PdO-CuO/Al2O3): Efficient Electrodes for Heterogeneous Catalysis of Ammonia Electrooxidation

RuO2 and PdO promoted CuO/Al2O3 mixed metal oxide nanostructures synthesized via co-impregnation technique demonstrated a robust electrocatalytic activity towards ammonia electro-oxidation (AEO). Physio-chemically characterized RuO2-CuO/Al2O3 and PdO-CuO/Al2O3 catalyst powders exhibited high electrochemical active surface area (ECSA), highest being shown by 1% RuO2-CuO/Al2O3. All fabricated electrodes displayed pleasing electroactive response towards AEO but ternary metal oxides showed superior and promoted catalytic output owing to their larger ECSA, higher current densities (j), larger diffusion coefficients (D°), greater apparent rate constants (kapp), and lower charge transfer resistance (Rct) values. Lower values of Rct obtained from EIS indicated the facilitation of electro-oxidation process over modified electrodes. The finest catalysis performance is expressed by 1% RuO2-CuO/Al2O3 producing the maximum oxidation current. All the promoted CuO/Al2O3 mixed metal oxide exhibited excellent stability and reproducibility in analyte solution. Consequently, γ-Al2O3 supported metal oxide electrocatalysts can be of great socio-economic significance by enhancing the energy production kinetics thereby achieving a higher energy output.

Electrooxidation of ammonia on A-site deficient perovskite oxide La0.9Ni0.6Cu0.35Fe0.05O3-δ for wastewater treatment

Wastewater can contain high amounts of ammonia which can pose as a great safety threat if released into natural waters. The electrochemical oxidation of ammonia offers a viable strategy to remove high concentrations and provides an attractive method for wastewater treatment. However, finding a highly efficient and low-cost catalyst is imperative for overcoming the sluggish nature of ammonia oxidation reaction. Herein, a modified A- and B-site perovskite is proposed as a catalyst for the oxidation of ammonia, making it suitable as an anode in an ammonia electrolyser. A series of La1-yNi0.6Cu0.4-xFexO3-δ (x = 0, 0.05 and 0.10; y = 0, 0.05 and 0.10) perovskite materials were synthesised by a conventional sol–gel method. Amongst those tested oxides, La0.9Ni0.6Cu0.35Fe0.05O3-δ was found to have superior activity towards the electrooxidation of ammonia due to an optimised amount of Fe doping and the presence of oxygen vacancies introduced by an A-site deficiency. Subsequently, La0.9Ni0.6Cu0.35Fe0.05O3-δ was employed as an anode in an ammonia electrolyser where the ammonia removal efficiency reached 95.4 % in simulated wastewater after 80 hr and a substantial reduction in real wastewater was also observed. These results demonstrate that the A-site deficient perovskite materials are a viable electrode for the removal of ammonia in a practical energy setting and paves way for future applications.

Platinum nanoparticles loaded on pristine and boron oxide modified carbon nano-onions for enhanced ammonia electrooxidation in alkaline direct ammonia fuel cells

Well dispersed ultrafine platinum nanoparticles (Pt NPs) loaded on pristine (p-CNOs) and boron oxide modified carbon nano-onions (B-CNOs) were successfully synthesized by the ethylene glycol reduction method. The obtained materials showed the presence of the face centered cubic structure of Pt with an average particle size of 4.6 ± 0.9 nm (p-CNOs) and 4.4 ± 0.5 nm (B-CNOs) under the alcohol reduction conditions. Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of C, O, B, and Pt on the surface of the CNOs. Transmission electron microscopy images showed that the addition of B and Pt onto the surface of CNOs did not alter their morphology. The electrochemical catalytic performance of Pt/p-CNO, unwashed (uw) Pt/B-CNO, washed (w) Pt/B-CNO and commercial Pt/C nanocatalysts were evaluated towards ammonia oxidation in alkaline electrolyte. The Pt/B-CNOuw hybrid nanocatalysts showed enhanced catalytic activity, better stability, and fast charge transfer for ammonia oxidation reactions as compared to Pt/B-CNOw, Pt/p-CNO and the commercial Pt/C hybrids. The improved electrocatalytic activity of the Pt/B-CNOuw hybrid is ascribed to an increase of the electronic conductivity of the carbon nano onions by boron oxide and the enhancement of the interaction with water due to the boron on the support, which could contribute to the oxidation of intermediate products from the ammonia electro-oxidation.